EVM - Error Vector Magnitude

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EVM (Error Vector Magnitude) is a measure of how accurately a radio is transmitting symbols within its constellation. EVM is a measure of signal quality, which is a function of noise, interfering signals, nonlinear distortion and the load of the radio. It is a component of the 802.11 IEEE standard, and has become an industry standard measurement for cellular phones, cable television and wifi. EVM is typically measured in decibels (dB), and sometimes in percent. An example will make this is clear, assuming you know a bit about digital modulation techniques (QAM, QPSK, PSK, etc).

Suppose our radio is transmitting via a 16-QAM constellation. It would like to send the black dots below in the I-Q (In phase - Quadrature Plane) plane. However, due to our real-world (non-ideal) radio, suppose the radio actually transmits something a bit off of this point:

16 qam constellation with no EVM

Figure 1. Illustration of A 16-QAM Constellation.

In Figure 1, we have a 16-QAM constellation, which means we encode our 1's and 0's as 16 different symbols, with 4 bits per symbol. At this instant in Figure 1, suppose we are transmitting the symbol pointed to by the orange vector, or bits [0000]. In this case, we are transmitting exactly what our radio wants to transmit; simiarly this is what the receiver would expect to receive with no noise present.

Now, suppose that our radio is not perfect for whatever reason. Then we won't be exactly transmitting the symbol we want to send. The difference between the desired (ideal) signal vector and the actual signal vector is the error vector, as shown in Figure 2. And the magnitude of the error vector? This is EVM.

error vector magnitude

Figure 2. Illustration of The Error Vector Magnitude (EVM).

Now, if you have noise in your system, this disturbs your measurements as well. However, EVM is not noise. Noise arises from some external source and can be reduced via averaging or other techniques. We'll return to what causes EVM in a minute.

EVM is typically measured in dB, as in: EVM=-28 dB. This means the error vector has a magnitude that is 28 dB less than the average signal vector (or, the average energy per symbol we transmit). Hence, we can write EVM mathematically as:

error vector magnitude equation

EVM is typically less than -20 dB, and often much lower depending on the application.

How does EVM relate to Antennas?

As this website doesn't focus too much on radios, you may be wondering why the topic of EVM is being studied in an antenna theory website. Well it turns out that the antenna can significantly affect EVM. How?

The antenna's impedance presents itself as a load to the radio. If the antenna has a poor impedance match, then it will have a high VSWR. This presents a difficult load for the radio to handle, causing a lot of power to be reflected to the radio. In this case, the radio may be jolted such that the signal quality of the radio degrades, which is measured by a higher than normal EVM. Hence, if an antenna engineer screws up the impedance matching, the radio (RF) team will quickly determine that the antenna is messed up, and will want to know why their EVM measurements are screwed up.

For this reason, it is important for antenna engineers to understand a bit about EVM, and why impedance matching on the Smith Chart is necessary.

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